Edit; now I went back and reread the question, I saw opens source. I will leave the comment as it could be a turnkey solution and stepping stone option to going full open source.

Something like a server on a raspberry pi could be what your after, but I don’t have specific software recommendations for it.

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I’ve just started dabbling, so don’t consider this an expert, but a suggestion.

Synology nas’s have a focus on data sovereignty . They have Android apps that communicate from your phone to the Nas for media.

They can also be setup as a private cloud drive initially accessible by VPN to access and a email / web interface as well as instant messager and webdrive

I gave some though to setting up a small private ‘corporate’ style server where people could access